Fluorescent lamps have been the commonest method of lighting consumer, commercial and industrial areas for many years. In operation, a gas mixture enclosed in the glass tube of the lamp is ionised by means of a high voltage pulse applied between two heated electrodes at each end of the tube. In a conventional lighting system, the gas in the fluorescent tube is extinguished and then ionised again with each half cycle of the 50 Hz conventional line frequency. This system has the merit of low capital cost and simplicity, but whilst far superior to incandescent lamps in the conversion of energy to light, it is nonetheless an inefficient mechanism. The circuit watt losses are similar whatever the wattage of the lamp and range from about 66% for an 18 watt lamp to 20% for a 70 watt lamp. In addition, the flicker caused by the re-ionisation of the lamp every half cycle at 50 Hz is now recognized as a major cause of headaches amongst office workers.
In consequence, a number of improvements have been initiated over the years to reduce the inefficiency and the flicker associated with fluorescent lamps.
An electronic controller addresses a number of these problems. It supplies the gases in the tube with a high frequency AC current, preferably above 18 kHz. This type of controller typically reduces circuit losses from the range 20-66% to the range 4-8%. Owing to the high frequency refresh rate of the lamp, its light output is increased. Accordingly, lamps are commonly under-powered such that the same output is produced as that resulting when running the lamp with a standard mains frequency circuit. For example:
Standard Circuit Lamp wattage: 18 watts Circuit losses: 12 watts Total power consumption: 30 watts High Frequency Circuit Lamp wattage: 16 watts Circuit losses: 2 watts Total power consumption 18 watts
The lumens output of each lamp in the above example would be identical.
The use of a high frequency controller is also beneficial as the refresh rate of the lamp is effectively 60,000 a second when running at 30 kHz, for example. Therefore, there is no flicker detectable by the human eye. Also the electronic controller unit can be less than half the weight of a standard circuit, and generate less heat. An electronic controller is also more versatile. For example, it can be interfaced with passive infra-red movement detectors or optical sensors which detect ambient light levels.
It is generally desirable to include a dimming facility in a lighting system, as the required lighting level may vary depending on various factors. For example, an office may be converted to intensive computer use, and a lower level of lighting is then appropriate owing to the relative dimness of a computer screen. Also, it has been found that the light tolerance and the amount of light needed or felt to be needed for given tasks varies greatly between individuals. In particular, it varies considerably between different age groups. 50-60 year olds will require substantially more light for the same range of tasks as 18-25 year olds. In addition, the light required within open plan and cellular offices varies greatly according to the type of partitioning system, colours and furniture used. Furthermore, office layout designs are changed frequently and in large organisations this can affect as much as 20% of the office space per annum. In consequence the original lighting can be either too bright or too dim in the revised spatial layout.
The abstract of JP-A-01084596 describes control circuitry for a discharge lamp, in which a variable inductance is provided to control the light output of the lamp.
Various forms of dimmable high frequency electronic controllers are available which can reduce their operating wattage from 100% to about 5%. Typically, a wall mounted potentiometer operable by a user is provided to send a control signal to each controller. Each controller accordingly alters the current and frequency which powers the discharge lamps of the respective luminaire. However, such controllers are expensive, typically costing 60% more than a conventional electronic controller. Therefore, it is only worthwhile to link at least ten and usually at least twenty-five luminaires in the dimming circuit, such that light levels can only be adjusted over large areas and in a uniform manner. Furthermore, in such a configuration, wiring needs to be routed from the wall mounted potentiometer to each luminaire in turn to carry the control signal. Installing this wiring is a time-consuming process, particularly when refurbishing a building having existing partitions, fixtures and the like.